Many aspects of embryonic epithelial-mesenchymal crosstalk are redeployed in adult skin where hair follicles cycle through periods of active growth (anagen), periods of regression (catagen) and rest (telogen) (Fig. 2). During anagen, matrix cells enrobing the dermal papilla continue to proliferate and differentiate, elongating the emerging hair shaft. Matrix cells have a finite proliferative capacity, influenced in part by the size and stimulatory output of the dermal papilla. 84 , 86 When the proliferative potential of matrix cells is exhausted, a destructive phase ensues resulting in apoptotic cell death of the lower two-thirds of the hair follicle. 87 During catagen, the dermal papilla also dramatically reduces in size and transforms into a quiescent cluster of cells. 88 However, the shrunken dermal papilla is pulled through the dermis trailing the lower portion of the follicle as it regresses to a resting position close to the permanent bulge region, the niche of epidermal stem cells. 86 , 89 The resting telogen hair follicle remains essentially inactive, with a small ORS-like hair germ and rudimentary dermal papilla, separated by a thick basement membrane. 90 Having lost its anchorage, the old hair shaft, or hair club, may be readily shed. 87 Although the exact nature of the trigger to enter the next anagen cycle remains unknown, it is hypothesized that in response to signals from dermal papilla, epidermal stem cells in the bulge region are transiently activated to proliferate and repopulate the lower portion of the hair follicle (“bulge activation hypothesis”). 86 , 91 Subsequently, dermal progenitors are recruited to the dermal papilla from reservoirs in the adjacent dermal sheath 88 likely in response to epidermal signals from secondary hair germ. As anagen proceeds, a similar cascade of mesenchymal-epithelial cross-talk that leads to the production of matrix cells, inner root sheath and hair shaft lineages during embryonic hair follicle morphogenesis is redeployed
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=eurekah.section.62117
